regulator: Rename s5m8767_convert_voltage to s5m8767_convert_voltage_to_sel

This function finds the smallest voltage that falls within the specified range,
and then returns the selector. This rename makes the intention more clear.

Also remove unneeded local variables min_vol and max_vol in s5m8767_set_voltage
and s5m8767_set_voltage_buck.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sangbeom Kim <sbkim73@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>